1. Field of the Invention
The present invention relates to a method for determining pulmonary stress, as well as to a breathing apparatus for regulating a respiratory gas flow.
2. Description of the Prior Art
U.S. Pat. No. 4,351,344 discloses a method and apparatus for monitoring lung compliance. A constant flow of gas is supplied during inspiration and a pressure versus time relationship is recorded. The pressure-time relationship is analyzed with respect to linearity. More specifically, the temporal length of a linear slope segment in the pressure-time relationship is determined. The temporal length can be compared with limits and an indication of the compliance status for the patient can be made based on the comparison.
The information thus obtained is, however, insufficient and inconclusive for being properly used in determinations of the status of the lung and as a tool for improving treatment of a lung.
Mechanical ventilation is used as a life saving treatment in many circumstances, but it can aggravate pre-existing disease and even induce lung injury if the dynamics and physiology of mechanical breath delivery are not considered. The lung has an inherent tendency to collapse. During normal breathing this tendency is counteracted by the chest wall and a natural substance called surfactant.
In disease the collapsing tendency becomes more pronounced, giving rise to areas (alveolar units) collapsing early during exhalation/expiration and opening late during inhalation/inspiration. This cyclic opening and closing of airways may initiate lung injury manifest as gross air leaks, diffuse alveolar damage, pulmonary edema and pulmonary inflammation, all of which have been termed Ventilator Induced Lung Injury (VILI). The cyclical opening and closing of alveolar units can be counteracted by the administration of a correctly set Positive End Expiratory Pressure (PEEP).
A second postulated mechanism for VILI is the delivery of large tidal volumes (which can cause volutrauma) or high end inspiratory airway pressure (which can cause barotrauma). Both may over-stretch lung tissues, leading to fluid accumulation, inflammation and increased stiffness of the lung. Baro-volutrauma can be avoided by setting a proper tidal volume or peak pressure.
If the ventilator settings are not optimized, the period before VILI is manifest can be considered as a period of increased stress. Hence, a determination of the degree of lung stress that may follow from a specific ventilator setting can be considered as a pulmonary stress index (PSI).
In previously filed but not published Swedish Patent Application No. 9904643-5 (published on Jun. 20, 2001 as European Application 1 108 391 A2), corresponding to co-pending U.S. application Ser. No. 09/736,346 filed Dec. 15, 2000, a method and apparatus solving these problems is disclosed. The method described in this application is based on P-t measurements made during inspiration.
It is an object of the invention to provide an alternative method for assessing the pulmonary stress.
This object is achieved in a method that includes obtaining a pressure-volume relationship based on a gas flow received from the lungs of a passively exhaling subject. As disclosed in the previously filed application, this is essentially the same as a P-t relationship since volume is the integral of flow over time. By analyzing the profile of the resulting P-V relationship, essentially the same information can be extracted as in the previously filed Swedish Patent Application mentioned above.
One advantageous analysis is obtained by adapting the profile to a power equation, e.g. in the form of P=a*Vb+c, where P is pressure, V is volume and a, b and c are constants. Determination of constant b is particularly interesting since b is a determinant of the shape of the profile. If b equals 1, the profile consists of a straight line, if b is less than 1 the profile is concave and if b is higher than 1 the profile is convex.
Convex profiles have been found to correspond to risks of progressive over-distension of lungs (decreasing compliance) and concave profiles have been found to correspond to risks associated with cyclic closing and opening of alveolar units (increasing compliance). Profiles also can be sigmoidal, i.e. include both concave and convex portions.
Analysis can be performed on pressure-volume relationship on a breath-by-breath basis or on averaged values over a plurality of breaths.
Another advantageous analysis is obtained by adapting the profile to a polynomial equation, or other mathematical expression providing an indicator of convexity or concavity.
The above object also is achieved in a breathing apparatus for implementing the above-described method.
Basically, the apparatus has a gas regulator for regulating respiratory gas flows, a pressure gauge for (directly or indirectly) measuring a pressure, preferably the airway pressure and a control unit for controlling the gas regulator. A meter or unit for determining exhaled volume is also included in the apparatus. The control unit is further adapted to perform the methods described above.
In a preferred embodiment, the control unit is adapted to compare the constant b with an interval, preferably with a lower limit between 0.5 and 0.95 and an upper limit between 1.05 and 1.5. As long as the constant b falls within the interval, there is no pulmonary stress. If the constant b falls outside the interval there is pulmonary stress. The value of the constant b thus provides both an indication of the presence of pulmonary stress and the magnitude of it. The constant b can therefore be used as a value for pulmonary stress index, PSI.
Similar results are obtained when other mathematical expressions are used.
In another preferred embodiment, the apparatus has a display unit and an alarm unit. The control unit is further adapted to perform at least one of a number of actions depending on e.g. the value of the constant b (pulmonary stress index). The control unit can generate an alarm when the stress index is too high or too low, indicating that a possibly injurious therapy is being delivered to a subject. The control unit can display the stress index, as well as the P-V relationship, on the display unit. The control unit can calculate suitable changes in control parameters for reducing pulmonary stress and display these as options for an operator on the display unit. It can automatically re-set the control parameters in accordance with calculations of suitable changes in the control parameters. The control unit can determine if recruiting maneuvers should be provided and can recommend or automatically perform such recruiting maneuvers or actions.
The apparatus according to the invention can advantageously be used for automatic re-setting of PEEP, tidal volume, airway pressure, I:E ratio or other ventilator-controlled parameters.